Tunable high-frequency tank circuits



Sept. 30, 1947. w, Q Q T A 2,428,037

Filed Jan. 22, 1943 INVENTORS [#14 75/? KIN 5. $05557 ATTORNEY PatentedSept. 30, 1947 TUNABLE L'HIGH-FREQUENCY TANK CIRCUITS Walter'van B.Roberts and Samuel G. Frantz, Princeton, N. J assignorsto RadioCorporation of .America, va corporation :of Delaware Application January22, 194:3,Serial'No. 473,196

The present inventionrrelates: to --.tuned circuits and; moreparticularly, to high frequency'tanks in the form "of resonant chambers.Iris-known that at ultra high frequencies-a tuned'circuit, in-

cluding a conventional'coilandcondenser, has a comparatively low factorof merit or' Q. In .order toobtain an improved factor of merit astructure in the form-of a: single turn toroid. winding with the tuningcapacity in the form of fiat plates at the center of the toroid may beused..

Qoptimum="89R\/ -F where R' is the :radius: of. revolution. of the torusin. centimeters and F is: the.- frequency in megacycles. The radiusr-ofrthe-generating circle of the torus must be equal*to .707 Rior theabove equation to hold. This relation was given by H. G. Clavier in.Communications magazine, September 1942-. It will'benoted thatit isdesirable to makeptheresonant structure quite large but practicalconsiderationsiusually place-a limit on. size.

An object of. the present invention is: thezconstruction of a high. Qtank. circuit which is readily tunable.

A further object of the present invention is the provision of stableadjusting means for tuning a toroidal, resonant circuit for highfrequencies,

Still. a further object of thepresent invention is the provision ofeasily adjusted means for tuning a toroidal tank circuit.

Still a further object ofthe present inventionis the provision of. ahigh "Q tank circuit which may be easily constructed.

Still a further object of the present invention is the provision of ahigh frequency tank circuit which may be designed to respond in apredetermined manner to certain. variations in meteorological.conductions, such as temperature or pressure.

The, foregoing objects, and others which. may appear from the followingdetailed description, are attained by providing a tank circuit in theform of a conductive Walledv chamber having characteristics such thatthe spacing between a pair of opposing walls determines the resonantfrequency. The spacing is adjusted by providing a conductive rod betweenthe Walls for pulling them together orforcing them apart. The length ofthe rod is so chosen that it has a very small efiective conductance atthe operating frequency. Specifically, the tank circuit may be a singleturn toroidal Winding having a tuning capacity in the form of a fiatplate at the center of the toroid.

Extending through the center of the flat plates 11 Claims. (01; 178-44)is a'rod having such length as to present a relatively high inductanceto the waves within the resonant chamber, said rod being threaded at itsends: and arranged to vary the spacing between the plates as a result ofit's rotation,

The present invention also contemplates hermetically sealing theresonant chamber, as described above, so that its dimensions willchangein a predetermined manner with variation in atmospherie pressure.Alternatively, the tuning structure may be so constructed as to have anextremely high coefficient of expansion with temperature changes sothatan enormously increased variation in frequency is attained formoderate variations in temperature.

'The novel features which, it' is believed, are characteristic of thepresent invention are pointed out with particularity in the appendedclaims. The invention will, however, be more completely understood byreference to the following detailed description, which is accompaniedby'a drawing inwhich Figure 1 illustrates in perpective, and partly insection, a high frequency tank circuit of a type to whichthe presentinvention may be applied; Figure 2 shows, likewise in perspective andpartly in. section, the application of the present invention to the tankcircuit of Figure 1, while Figure 3 illustrates a modification of'theform of the invention shown in Figure 2.

The high frequency tank circuit of Figure 1 includes an, inductance inthe form of a single toroidal chamber 5: having an annular slit 6 alongthe inner surface of the torus. Across the edges of the slit 6 areconnected a pair of circular plates "8' and 9 providing a tuningcapacity for the tank circuit, the inductancev being provided by theinterior circumferential length of chamber 5 between one edge ofslit 6and the other edge. Energy is applied to or abstracted from th tank cir-'cuit by means of transmission line TL, shown in this figure asincluding a pair of parallel conductors l0 and H. The conductors Ill andII terminate within the interior of the torus in a coupling loop l2, thedimensions'of which may be adjusted, as desired, to vary the couplingbetween the transmission line and the resonant circuit. The conductorsofv the transmission line may pass through apertures in the wall of thechamber or the apertures may be sealed by insulators l3 if it is desiredto make the tank circuit airtight.

As mentionedabove, the-resonant frequency of the tank circuit is alteredWithin certain limits by varying the spacing between capacity plates 8and '9; Heretof'ore, the most practicable Way of doing this has been toprovide a clamping structure, such as C clamp [5 having an adjustingscrew l6 for pressing plates 8 and 9 closer together or allowing them tospring further apart due-to the spring action of the toroidal portion 5.A practical objection exists to this method of tuning in that therelatively long dimension of G clamp 15 are likely to be considerablyvaried by temperature variations unless the entire C clamp is made of amaterial which is substantially nontemperature responsive. However, suchmaterials are relatively expensive and large quantities are necessarydue to the size of clamp Hi.

In accordance with the present invention, as shown in Figure 2, animproved means is provided for adjusting the spacing between plates 8and 9. The plates are made of thick material so as to be relatively freeof microphonic vibration and are squeezed together by rod 18 passingeach half may be spun from a single sheet of metal. The only jointl irithe structure is the solder joint 33 around the circumference of thedevice. The details of operation of the tuning means I8 is the same asdescribed with reference to Figure 2.

In the modifications of the invention shown in Figures 2 and 3, in orderto prevent variations in atmospheric pressure from varying the tuningthrough the space between plates 8 and 9. In

portions 22 and 23 of rod l8." It is contemplated as being within thescope of. the present invention to make each of the pipes 19 and 26equal in length to substantially 'one quarter of the mean operatingwavelength of the system so that only a very small effective conductanceis shunted across between plates 8 and 9. Thus substantially no energyis absorbed by the adjusting means for tuning the tank. The rod l8,together with the surrounding pipes l9 and 20 will preferably, in thisform of the present invention, be adapted to have substantially zerotemperature coefficients. For example, they may be made of steel whichhas substantially a zero temperature coefficient and is silvered orgold-plated in order to provide greater conductivity on the currentcarrying surfaces.

The threaded portions 22 and 23 of rod [8 may be threaded in oppositedirections as is done in turnbuckle construction if only coarsefrequency adjustment is desired. However, if it is desired to obtain avery fine adjustment of the operating frequency of the tank circuit thethreads 22 and 23 are preferably cut in the same direction but withslightly different pitches so that several turns of rod 18 arerequiredto produce a very small change in the separation between plates 8 and 9.A suitable dial and knob arrangement 24 may be provided at one end ofrod l8 for the purpose of recording the adjustment of the tank circuit.This dial arrangement may be a more or less conventional tuning dialarrangement or it may be constructed in the form of a micrometer head. Acoupling to the tank circuit of Figure 2 is provided by means of loop l2connected to transmission line TL asin the case of Figure 1. However, ifdesired, an additional or alternate coupling may be obtained by tappingconductor 26 of a coaxial transmission line CTL to a desired point alongrod 18. Impedance matching is provided by varying the point of contactof conductor 26 with rod [8.

The modification of the present invention shown in Figure 3 is similarin operation to that shown in Figure 2 but the structure is moreadaptable to construction by a spinning process. For convenience inconstruction the tank is divided laterally into two shell-like halves 30and 3!. Each of the halves may be separately spun and then assembled andsoldered together, as indicated by the solder joint 33. The pipes l9 and20 of Figure 2 are replaced by somewhat hemispherical portions 32, 34 inFigure 3. Thus of the tank, a small air leakage should be provided atsome point. This may be at the point of insertion of the transmissionline, or at some other point. However, advantage may be taken of theefiect of variation in atmospheric pressure upon the frequency of thetank for the purpose of measuring elevation by means of soundingballoons. In this case the tank circuit would be employed as thefrequency determining element of an oscillator, the input and outputcircuits of the tube being coupled to the tank by coupling links such asloop 12 and/or tap 26. The coupling links should pass through airtightinsulating seals such as insulators l3,' l3 of Figure 1. If the tanktuning means is made of zero temperature coefiicient material thefrequency of the tank will then be a function of atmospheric pressureonly and a receiver located on the ground to measure the frequency ofoscillations emitted by the transmitter carried by the sounding balloonwill obtain therefrom, in' conjunction with a calibration, curve, theatmospheric pressure at the location of the balloon. If desired, anadditional transmitter using a similar tank circuit as a frequencydetermining element may be used in which the tuning rod l8and the pipesl9 and 20 are made of materials having widely different temperaturecoefficients so that'the generating frequency will vary as a function oftemperature; Thus a separate receiver located on the ground to measurethe frequency of operations emitted from the second oscillator willobtain therefrom, in conjunction with a. calibration curve, theatmospheric temperature at the location of the balloon. The secondmentioned tank should, of course, not be hermetically sealed as,otherwise, the variation in pressure will also influence the generalfrequency. 7

While we have illustrated some specific embodiments of the presentinvention it should be clearly understood that the invention is notlimited thereto since many modifications may be made in the severalelements employed and in their arrangement and it is, therefore,contemplated by the appended claims to cover any such modifications asfall within the spirit and scope of the invention.

We claim: g

1. A tank circuit including inductance and a pair of plates providingcapacity connected across said inductance, and means for varying thespacing between said plates comprising a metallic adjustable spacingbar, said bar being arranged to act as a low loss shunt inductanceconnected between said plates, said bar being threaded through and inconductive connection with each of said plates.

2. A tank circuit including inductance and a pair of plates providingcapacity connected across said inductance, and means for varying thespacing between said plates comprising a metallic adjustable spacingbar, said bar being arranged to act as a low loss shunt inductanceconnected between said plates, said bar being threaded through and inconductive connection with each of said plates, the threads being inopposite directions.

3. A tank circuit including inductance and a pair of plates providingcapacity connected across said inductance, and means for varying thespacing between said plates comprising a metallic adjustable spacingbar, said bar being arranged to act as a low loss shunt inductanceconnected between said plates, said bar being threaded through and inconductive connection with each of said plates, the threads being in thesame direction and of different pitches.

4. A resonant circuit including a conductive walled resonant chamber,the operating frequency thereof being determined by the spacing betweena pair of opposing walls and means for adjusting said spacing includinga conductive rod passing through said chamber from one of said walls tothe other, the length of said rod being such as to present a highimpedance at the operating frequency.

5. A resonant circuit including a conductive walled resonant chamber,the operating frequency thereof being determined by the spacing betweena pair of opposing walls and means for adjusting said spacing includinga conductive rod passing through said chamber from one of said walls tothe other, the length of said rod being such as to present a highimpedance at the operating frequency and means for varying the point ofattachment of at least one of said walls to said rod.

6. A resonant circuit including a conductive walled resonant chamber,the operating frequency thereof being determined by the spacing betweena pair of opposing walls and means for adjusting said spacing includinga conductive rod passing through said chamber from one of said walls tothe other, the length of said rod being such as to present a highimpedance at the operating frequency, said rod being threaded throughsaid walls, the threads passing through said walls being in oppositedirections.

7. A resonant circuit including a conductive walled resonant chamber,the operating frequency thereof being determined by the spacing betweena pair of opposing walls and means for adjusting said spacing includinga conductive rod passing through said chamber from one of said walls tothe other, the length of said rod being such as to present a highimpedance at the operating frequency said rod being threaded throughsaid walls, the threads being in the same direction but of differentpitch.

8. A resonant structure including a pair of complementary circularplates each having a concavity in the center, a flat annularintermediate portion and a semi-circular trough around the edge andmeans for securing the outer edges of said troughs together to form atoroidal inductance with the flat portions of said plates cooperating toform' a tuning capacity for said inductance, there being threadedapertures at the center of each concavity to accept a threaded rod foradjusting the spacing between the fiat portions of said plates.

9. A resonant structure including a toroidal conductive structure havingan annular slot on the inner surface thereof, flat conductive parallelplates connected to adjacent edges of said slot, each of said plateshaving a tubular member attached to its midpoint, said tubular membersbeing directed in opposite directions and closed at their opposite ends,and a conductive rod passing through said tubular members from one endto the other and threaded through said closed ends.

10. A resonant structure including a toroidal conductive structurehaving an annular slot on the inner surface thereof, fiat conductiveparallel plates connected to adjacent edges of said slot, each of saidplates having a tubular member attached to its midpoint, said tubularmembers being directed in opposite directions and closed at theiropposite ends, and a conductive rod passing through said tubular membersfrom one end to the other and threaded through said closed ends, thelength of said rod being such as to present a high inductance to energyof the operating frequency of said structure.

11. A resonant structure including a toroidal conductive structurehaving an annular slot on the inner surface thereof, flat conductiveparallel plates connected to adjacent edges of said slot, each of saidplates having a tubular member attached to its midpoint, said tubularmembers being directed in opposite directions and closed at theiropposite ends, and a conductive rod passing through said tubular membersfrom one end to the other and threaded through said closed ends, thelength of said rod being such as to present a high inductance to energyof the'operating frequency of said structure, each of said tubularmember having a length of the order of one quarter of the operatingwavelength.

WALTER. VAN B. ROBERTS. SAMUEL G. FRANTZ.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,737,741 Thomas Dec. 3, 19292,085,223 Kolster June 29, 1937 2,242,275 Varian May 20, 1941 FOREIGNPATENTS Number Country Date 244,009 Great Britain Dec. 10, 1925

